The present invention pertains to inductive components for use with integrated circuit devices. In particular, the present invention pertains to inductive components formed using fabrication techniques similar to those used in the fabrication of integrated circuit devices.
An increasing number of products use electronic circuitry formed in integrated circuits. These integrated circuits may replace circuits formed of discrete components mounted on a board. In many circumstances, integrated circuits provide a variety of advantages over discrete components, such as smaller size, lower power consumption, faster performance, greater reliability, and lower placement and overall manufacturing costs.
The techniques for manufacturing integrated circuits, including the steps of laying down successive layers of material in particular patterns, are well-known in the semiconductor arts. For the most part, these integrated circuits have consisted of active components (principally transistors). More recently, a few types of passive components have been formed onto integrated circuits. However, these passive components have been largely limited to capacitors, resistors and a few types of inductive devices of low inductance, because of limitations imposed by conventional semiconductor manufacturing techniques.
The fabrication of high inductance inductive components by integrated circuit manufacturing techniques has presented significant technological challenges. One problem has been the cross-talk induced in buried active components by the fields created by overlying inductors. Another challenge has been the difficulty of forming a magnetic inductor core using batch semiconductor manufacturing processes.
Many types of electronic devices include a large number of inductive components. This is particularly true of radio frequency devices, such as radios and cellular telephones, which filter analog signals. To date, these products have had much of their circuitry formed in small integrated circuits, but have required large circuit boards to hold the discrete passive components, including resistors and capacitors, and particularly inductive components. Integrating the inductive components with the remainder of the electronic circuitry in integrated circuits would allow electronic devices to be much smaller than has previously been possible. Greater integration of the inductive components into the integrated circuits would provide more efficient utilization of space, lower power consumption, and would also provide greater reliability. Thus, there has been a long-felt need to integrate inductive components into integrated circuit devices.
Thus far, the techniques developed for integrating inductive components into integrated circuit devices have involved expensive processes that are not compatible with high volume mass production, or that have made significant trade-offs in terms of device performance and efficiency. For example, one technique for producing an inductive component on an integrated circuit device has been to form a planar inductive component comprising a single layer coil on the substrate. The size of such planar inductive components is limited, as is the number of turns that may be included, generally about 5 to 10 turns. The limitation on the number of turns in the coil limits the inductance and the quality factor ("Q") of the inductive component. Furthermore, such inductive components are generally limited to a maximum inductance of about 100 nanohenries. In addition, the magnetic flux lines of such an inductive component are primarily perpendicular to the plane of the integrated circuit device, forcing the magnetic fields into the substrate, resulting in possible cross-talk coupling with underlying devices, and possible energy losses due to interactions with the substrate itself.
Another problem in fabricating inductive components that may be included in integrated circuits is the need for relatively thick conductors that have consistent cross-sections, and consistent wire placement and spacing. These factors must be carefully controlled so that the inductive components may be repeatably manufactured.
In certain instances, thick film and micro-machining processes have been used to form inductive components as part of integrated circuits. See, for example, Park et al., "High Current Integrated Microinductors and Microtransformers Using Low Temperature Fabrication Processes", Microelectronics International, Vol. 14, No. 3 (September 1997); Ahn, "Micromachined Components as Integrated Inductors and Magnetic Microactuators", Chapter 2, Ph.D. Dissertation, Georgia Institute of Technology (May, 1993); Lochel et al., "Micro Coils Fabricated by UV Depth Lithography and Galvanoplating", Proceedings of the 8th International Conference on Solid State Sensors and Actuators, pp. 264-267 (June, 1995); Yamada et al., "Fabrication of Wrapped Micro Coils Wound around a Magnetic Core", id. at pp. 272-275; Watanabe et al., "A New Fabrication Process of a Planar Coil Using Photosensitive Polyimide and Electroplating", id. at pp. 268-271.
A thin film process for the fabrication of inductive coils is disclosed in U.S. Pat. No. 3,614,554. The metal layers formed by this process are necessarily quite thin in vertical cross-section, and thus current-carrying coils offer high resistances, while magnetic cores have high reluctances. The process requires the use of through-holes ("vias"), which increase the electrical resistance and thus power loss.
A batch fabrication process has recently been developed for forming an inductive component on a device containing an integrated circuit, using micromachining technology that is common to the semiconductor integrated circuit industry. The inductive component includes a substrate that forms a structural base. A lower set of spaced apart conductors is deposited and patterned on the substrate by conventional photolithographic and etching techniques. Deposition may be performed by means such as sputtering, electron beam evaporation, filament evaporation, or electrodeposition, depending on factors such as the type of metal used for the conductors. A lower insulation layer is deposited over the lower set of conductors so that the ends of the individual conductors are exposed. The lower insulation layer may be formed of polymeric insulating material, such as a thermoset polymer or thermoplastic. The lower insulation layer separates the lower conductors from a magnetic core element that is formed, by micro-molding technology, on a metallic seed layer (typically copper or gold) that is deposited on the lower insulation layer. An upper insulation layer is deposited over the magnetic core element and is also patterned to expose the ends of the lower conductors. This may be referred as a "via-less" architecture, since a separate patterning step is not required to create the vertical contact between layers. An upper set of spaced apart conductors is deposited on top of the second insulation layer so that the upper conductors contact the exposed ends of the lower conductors. The conductors of the upper and lower sets of conductors are interconnected to form one or more continuous coils around the core element. The entire structure may be encapsulated by a passivation layer for environmental protection.
The above-described inductive component may not be suitable, in certain applications, for construction on semiconductive substrates, such as doped silicon, due to electromagnetic coupling to the substrate or to nearby components and circuits. Furthermore, the via-less architecture creates a device with a relatively high vertical profile, thereby limiting the thickness of the magnetic core. Moreover, the magnetically induced currents in the substrate result in energy loss and quality factor ("Q") degradation.